[BE] Testing docs: clarify test instantiation function usage (#100905)
Beefing up docs with discussion about when to use `instantiate_device_type_tests()` vs. `instantiate_parametrized_tests()` + description on what each does.
Spoiler: use only one - the former for device-specific and the latter for device-agnostic tests. Both support `@parametrize`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100905
Approved by: https://github.com/janeyx99
